A number of conventional solid state lasers are based on inorganic semiconductors (e.g., GaAs) and typically are electrically driven. That is to say, the lasers (generally referred to as diode lasers) utilize recombination of injected electrons and holes in a laser cavity. This necessitates a design which has to meet both electrical and optical requirements. The requirements are relatively easily met in the case of inorganic semiconductors, where high carrier mobility allows utilization of relatively thick layers without too large penalties in terms of voltage drop and ohmic losses. Furthermore, the availability of closely related semiconductor materials with significant difference in refractive index facilitates easy fabrication of planar waveguides or distributed Bragg reflector-based vertical cavity lasers. These favorable properties have led to the rapid evolution of inorganic semiconductor lasers. For background on semiconductor lasers, see for instance, G. P. Agrawal et al., "Long-Wavelength Semiconductor Lasers", Van Nostrand Reinhold, New York 1986.
Despite the widespread use of inorganic semiconductor diode lasers, it would be of interest to have available lasers based on organic materials, since at least some organic materials have properties which could make them suitable as, for instance, gain media in very low threshold lasers. These properties include a Stokes shift between the absorption and emission bands, and the ability to easily realize guest-host systems in which a small volume fraction of the emitting species is embedded in a suitable host. However, while it is possible to realize low threshold organic lasers by optical pumping with radiation from a suitable external source (see, for instance, M. Kuwata-Gonokami et al., Optics Letters, Vol. 20 (20), p. 2093 (1995)), a number of factors have, to the best of our knowledge, so far prevented successful design of an electrically pumped organic laser. On the other hand, electrically pumped lasers generally are more convenient than optically pumped lasers. For instance, electrically pumped semiconductor lasers are generally more compact and easier to modulate than optically pumped semiconductor lasers. Thus, it would be desirable to have available organic lasers that do not require exposure of the laser to externally-created pump radiation.
By "conventional" electrical pumping we mean herein pumping by injection of electrons and holes into a diode structure, with recombination in a substantially undoped active region.
Regarding the use of organic materials in electrically pumped lasers, there are a number of difficulties that would have to be overcome. Among these are the very low carrier mobility in relevant materials, and the present unavailability of suitable combinations of materials with appropriately large refractive index difference and requisite carrier transport properties. These difficulties for instance make the design of a low-loss cavity (a prerequisite for low threshold lasing) a formidable task.